This invention relates to a pressure control system and a method of controlling the curing of certain resins, and adapting the control to the peculiarities of a batch and a particular cure schedule.
Graphite cloth reinforced PMR-15 resin is a high temperature composite material used in aircraft and automobile structures such as aircraft engine guide vanes. A given number of layers of graphite cloth impregnated with these polyimide resins are stacked and cured in a press or autoclave. At lower temperatures and pressures the solvents and water are cooked out and resin imidization takes place, and during a final stage the material is heated under high pressure to a temperature adequate to complete the cross linking reaction. Presently uncontrollable variations in the properties of the resins and/or of the manufacturing cure process used to convert prepreg lay-ups to finished products make it desirable to have a pressure control that applies high pressure to the curing workpiece at a time that depends on the developing properties of the workpiece material. Since sensors are used to measure the material properties, the principal requirements on the control structure are that it be sensor based and provide adaptive control.
Although there is a large body of literature and some commercial practice using such controls and many of the applications involve models of the operation of the controlled plant or desired plant output, no satisfactory technique for curing high temperature composite structures having polymer resins has been developed until now.